1. Field of the Invention:
This invention relates to a brake fluid-pressure control valve responsive to deceleration, most suitable for an automobile, and more particularly to a brake fluid-pressure control valve responsive to deceleration in which the ratio of rear wheel brake pressure to front wheel brake pressure is so reduced at brake fluid pressures beyond a predetermined brake fluid pressure, namely a control start brake fluid pressure, as to prevent the rear wheels from locking.
2. Description of the Prior Art:
When an automobile is strongly or rapidly braked, it is decelerated at a high rate. At such a time, the center of gravity of the automobile moves realtive to the wheels by the inertia of the automobile, and the load applied to the rear wheels is reduced. Thus, the reaction force between the rear wheels and the road is reduced, so that the rear wheels are apt to lock.
In order to prevent the rear wheels from locking, a brake fluid pressure control valve is arranged in a rear wheel brake system. The brake fluid pressure control valve causes the brake fluid pressure applied to the rear wheels to increase at a lower rate than the other fluid pressure applied to the front wheels when a predetermined brake fluid pressure is reached.
However, when, the control start brake fluid pressure (i.e. the pressure at which the increase rate of the brake fluid pressure applied to the rear wheels starts to reduce), is held constant regardless of the charge condition or load factor of the automobile, it is difficult to always prevent an automobile charged with much variable load, such as a truck, from locking. The load applied to the rear wheels remarkably varys between the no-load condition and the full-load condition in the truck.
When the predetermined control start fluid pressure is the optimum for the automobile charged with no load, the brake fluid pressure applied to the rear wheels of the automobile charged with some load is lower than required, and sufficient brake force cannot be supplied to the rear wheels. When the predetermined control start fluid pressure is the optimum for the automobile charged with full load, the brake fluid pressure applied to the rear wheels of the automobile charged with no load is higher than required, and there is the fear that the rear wheels lock. Such a fluid pressure control valve is unsatisfactory.
In order to remove the above described defects, a fluid pressure control valve responsive to deceleration was proposed which utilizes the fact that a brake fluid pressure which imparts a predetermined deceleration to an automobile is proportional to the weight of the automobile. In the fluid pressure control valve, brake fluid pressure is detected at which the deceleration of the automobile reaches a predetermined valve, and a control start fluid pressure is determined in accordance with the detected brake fluid pressure.
The above-described fluid-pressure control valve responsive to deceleration comprises; a stepped pressure-regulating piston having a smaller area subjected to a brake fluid pressure from a master cylinder; and a larger area subjected to a brake fluid pressure to be applied to rear wheel brake cylinders, valve means for opening and closing a passage extending from the master cylinder to the rear wheel brake cylinders in response to the movement of the pressure-regulating piston; and spring means for urging the pressure-regulating piston in the direction to open the valve and determining a control start fluid pressure. The urging force of the spring means for urging the pressure-regulating piston is so designed as to be proportional to the brake fluid pressure at which the deceleration of the automobile has reached a predetermined value.
In the above arrangement, the urging force of the spring means is proportional to the brake fluid pressure at which the deceleration of the automobile has reached the predetermined valve. And the control start fluid pressure is proportional to the urging force of the spring means. Therefore, the ratio of the control start fluid pressure to the brake fluid pressure at which the deceleration of the automobile has reached the predetermined value is constant regardless of the weight or load of the automobile.
However, as well known, the ratio of the load applied to the rear wheels to the load applied to the front wheels is smaller in the no-charge condition of the automobile than in the charge condition of the automobile. Accordingly, there is in the no-charge condition the possibility that the rear wheels lock at a lower decelration than a predetermined deceleration. On the other hand, the ratio of the load applied to the rear wheels to the load applied to the front wheels is larger in the charge condition of the automobile than in the no-charge condition of the automobile. Accordingly, there is little possibility in the charge condition that the rear wheels will lock even at a higher deceleration than the predetermined deceleration. However, a sufficient brake force cannot be always obtained.
Accordingly, it is desirable to determine the control start fluid pressure in accordance with the load applied to the rear wheels or the state of load of the automobile.
In other words, it is desirable that the brake fluid pressure control starts at a brake fluid pressure which corresponds with a lower deceleration occuring in an unloaded automobile, and starts at a brake fluid pressure which corresponds with an increased deceleration occurring with a loaded automobile.